Solution: Peano subtraction
from coursepy.math.peano import *
def pred(a):
assert peanop(a)
if zerop(a):
return zero()
else:
return a[0]
def peano_sub(a, b):
assert peanop(a) and peanop(b)
if zerop(b) or zerop(a):
return a
else:
return peano_sub(pred(a),pred(b))